I've found that a great way to crack your back, especially your lower back, into your hips is to lay on your stomach. Then, bring your left leg up - still on the floor, so that your knee is bent. This way, you will be laying kind of on your side, but mostly on your stomach. Then use your left hand to push on down on your buttocks while you try to pull the top portion of your body against it. Switch to the other side and repeat. It feels great.
i do something similar to that which i thought i invented but you beat me to it.
what i do is:
lay on your stomach, but kick one leg out front of you and have your other leg bent behind you, so you're on your stomach/side with your legs spread out. you must have both arms out in front of you, more importantly the one you're laying on. you should begin to see where this is going. then hold up your front leg as far in front of you as you can, and look behind you, and sort of stretch. then proceed to lift your front leg up and over, slowly rotating to your other side. your leg acts as a counter-weight for your body and you can stretch your back pretty easily. it sounds weird but it's so easy and it works.